Tidal and diel dynamics in a nursery area: Patterns in fish migration in a mangrove in north Brazil

Mangroves are considered important as fish nurseries worldwide. Block netting in intertidal mangrove creeks at HW revealed the overwhelming significance of the factors tide and time of day and their combinations (spring tide at dusk and dawn, neap tide at midday and midnight) in structuring fish assemblages. In contrast to neap tide and daylight when nekton catches were poorest, spring tide and darkness produced the most complex fish assemblage structure when abundant species occurred with increased proportions and more rare species entered. Intertidal fish were juveniles or adults of small species, or juveniles of larger species. Stomach analyses of fish showed that the intertidal movements are usually feeding migrations. Fish were concentrated in the subtidal sections at LW. Pioneer studies with a 200 kHz split-beam sonar beaming horizontally and vertically in a large mangrove channel, revealed that the entire fish community rode the first flood rise to achieve upstream transport and to enter the intertidal zone. Despite low Secchi depths, diel changes in vertical distribution were apparent when fish exploited the water column only at night. The four-eyed fish Anableps anableps entered the intertidal creeks rapidly with the first flood rise. They fed in the submerged mangrove at HW and returned gradually after the ebb current maximum to concentrate again in the subtidal parts of the main channel at LW. The combination of high inundation at daylight (spring tide-dawn) provided optimum foraging conditions for A. anableps while darkness and low inundation was linked to poorest foraging conditions (neap tide-night). The mangrove zooplankton was dominated by copepods. Zooplankton densities were higher at spring than at neap tide. In contrast to the number of zooplankton species, the densities were lowest at HW and highest at LW. Obviously, the temporal variations in abundance of fish and zooplankton are synchronized by the tide.